As the easily exploited hydrocarbon energy sources have been depleted, oil and gas wells have been drilled to ever deeper depths and have required more complex technology. Much of the current drilling activity is conducted from offshore drilling platforms which often support twenty or more wells. All but one of the wells drilled from such a platform are necessarily deviated from the vertical axis. Several methods for changing and controlling the direction of deviated or non-vertical boreholes have been developed and employed with varying levels of success and quality. One of the earlier and more successful interim methods was called a whipstock. The whipstock is basically a shaped body, generally iron or steel, placed in the existing borehole and oriented to deflect the drill into the desired direction. After the borehole is given this initial kick-off, a specially designed Bottom Hole Assembly (BHA) is used in an attempt to change the direction to the desired value. Multiple design changes are often required to get acceptable results. The BHA is then changed to a design intended to drill straight ahead. This whipstock method, as crude, inaccurate and cumbersome as it is, served the drilling for many years but is used less today. Another relatively old and useful method for changing and controlling the direction of a borehole is directional hydraulic jetting. In this method, the bit jets are arranged to produce eroding jet streams in an off-vertical direction while the drill is not rotating and the jet streams are oriented in the desired drilling direction. After a period of directional jetting, the drill is rotated to drill ahead a short distance. A series of such small steps can be used to turn to the desired direction. In soft formations, the jetting action is sufficient to cause drilling in the desired direction. This method is subject to the formation properties and prone to much trial and error.
Modern directional drilling practice generally employs downhole mud motors, a bend in the BHA or offset stabilizer, and a directional survey instrument to determine the direction of the bend. Commonly, the direction of the bend or offset is called Tool Face Orientation (TFO) and is determined either by gravity methods, or magnetic measurement. Today, this TFO information is generally provided in real time by either direct wireline or a Measurements-While-Drilling (MWD) system which most often uses mud telemetry.
There are two versions of the bend in the BHA. One is called a bent sub which is located above the drill motor. The location of the bent sub is too far from the bit to allow significant rotation of the drill string without causing undue stresses and component fatigue. Consequently, the use of the bent sub restricts drilling operations to substantially constant TFO. Thus the rate of curvature of the hole by this method is not dynamically controllable but rather is set by the BHA design and the drilling conditions. It is often necessary to make multiple trips in and out of the hole to change the BHA design until a satisfactory curvature is obtained.
The second version of the bend in the BHA is the so-called bent housing motor wherein there is a slight bend in the bottom section of the motor. This small bend in the motor causes a curvature in the hole in the direction of the bend much as in the case of the bent sub. The rate of curvature of the hole with constant TFO is a function of the bend and other BHA design factors along with borehole properties. Like the bent sub method, the rate of curvature of the bent housing method is not precisely controllable by design. However, the bent housing motor, due to its short bent section, can be rotated continuously or intermittently in the hole. By selective time sharing of the rotation and constant TFO operational modes, any value of average curvature between zero and the maximum value at constant TFO operation can be obtained. This basic capability reduces the number of trips into and out of the hole thus saving time over the bent sub method. However the quality of the hole drilled by this method suffers from the interleaving of the multiple straight sections and excessive curvature sections caused by this method.
The offset stabilizer method often used with turbine type downhole motors is similar to the bent housing system in that it will turn when a constant TFO is held and will drill straight ahead when the drill pipe is rotated. The turn is caused by the offset stabilizer putting a side force on the bit. The results are virtually identical with the bent housing motor system.
Most deviated wells drilled today are drilled basically in a two dimensional vertical plane from the surface location, most often an offshore platform, to the target location. Most such wells contain three distinct sections; a straight down vertical section, a build angle section in the desired direction, and a hold angle (inclination) straight section. Some wells also contain and additional drop angle section or a drop angle to vertical section and a bottom vertical section. Also horizontal webs are becoming popular wherein there is a long horizontal section that has near zero degrees inclination. The horizontal sections are generally in the producing zone for the purpose of enhanced production. When the producing zone is thin, very accurate directional drilling is required and almost always horizontal drilling increases the need for smooth, quality hole without excessive dogleg.
One of the most dominant features of a deviated well is the long hold section which follows the build section. The need here is to drill a quality hole straight ahead with minimal dogleg as quickly as possible. Standard rotary drilling wherein the bit is rotated by rotation of the entire drill string is the preferred method of drilling this section of the hole due to its higher penetration rate, higher quality of hole and long life of the components. The chief disadvantage of this method is that there is is no directional drilling method to control the azimuth of the borehole. So called packed hole assemblies (A BHA designed to drill straight ahead) are used in attempts to minimize the walk or wandering in azimuthal direction of the borehole with minimal success. Generally, multiple corrections of the borehole direction are required during this straight section. This is done by pulling the packed hole rotary drilling assembly from the hole and replacing it with a downhole motor and bent BHA to accomplish the directional correction. Then another trip is made to replace the rotary drilling assembly. This process must be repeated each time the direction of the borehole drifts too far from the plan. The bent housing downhole motor may be alternately used in this straight section at the sacrifice of longer times, higher costs and possibly higher dogleg hole. This higher dogleg effect is documented in the paper "First Real Time Measurements of Downhole Vibrations, Forces, and Pressures Used to Monitor Directional Drilling Operations", Cook, R. L. and Nicholson, J. W., SPE/IADC 18651, SPE/IADC Drilling Conference, New Orleans, La., Feb. 28-Mar. 3, 1989.
The latest technology in this area is represented by two technical publications by Henry Delafon: "BHA Prediction Software Improves Directional Drilling, Parts 1 and 2" Worm Oil March and April, 1989. Delafon demonstrates that in some environments sophisticated computer design of the BHA configuration can be used to reduce the number of direction corrections needed during the hold section using rotary drilling.
In view of this foregoing discussion, it is evident that a better and more efficient method of controlled directional drilling is needed. More specifically, it is apparent that there is a need to incorporate a method of directional control into standard rotary drilling which produces little or no interference with the optimal drilling efficiency of the rotary method. The directional rotary method described in greater detail below, provides a method and apparatus for continuously and automatically controlling the direction of an optimal rotary drill such that the borehole is drilled substantially along a preplanned profile with minimal dogleg in minimum time without tripping for directional purposes.